LEARNING HBOUT THE STARS 



175 



is 25,000,000,000,000 miles away, and about four and 

 one-third years is required for its light to reach us. 

 We know that the stars are gigantic self-luminous 

 masses of glowing solids, liquids, or gases. Many of 

 them are larger than our sun, which has a diameter 

 of 865,000 miles, but because of their great distances 

 from us they appear like sparkling jewels studded 

 in the sky. 



Courtesy Bausch and Lomb Optical Company 



FIG. 288. GREAT HERCULES STAR CLUSTER 



There are other bodies than stars in the heavens. 

 Planets are much smaller bodies that revolve around a 

 sun. Some of them look larger and brighter than stars 

 because they are much nearer to us. They are not self- 

 luminous but shine by reflected light from the sun. 

 You can distinguish a planet from the stars because 

 it gives off a steady light, whereas stars twinkle. Also, 

 planets change their position among the stars. Are 

 you acquainted with the planet Venus, sometimes 

 known as the morning or evening "star"? The moon, 

 meteors, and comets are still other heavenly bodies 

 that attract our attention, but they will be described 

 later. 



What is a light year? In our study of the heavens" 



we must become accustomed to thinking of great dis- 

 tances. Our unit of measurement of long distances 

 on the earth is the mile. Star distances are not gen- 

 erally given in miles because the figures are so large 

 that they cease to have any meaning to our minds. 

 Instead, the distance light travels in a year is taken 

 as the unit. Light travels about 186,000 miles a sec- 

 ond or about 6,000,000,000,000 (six trillion) miles a 

 year. The distance that light travels in one year is 

 the unit of measurement of distance in astronomy, and 

 it is called the light year. The distance to our nearest 

 star except the sun is about four light years. The 

 North Star is forty light years away, and there is evi- 

 dence that some of the most distant stars are at least 

 1,000,000 light years away. 



Courtesy Mount Wilson Observatory 



FIG. 289. A SPIRAL NEBULA, FULL VIEW 



What is the nature of the Milky Way? Have you 

 ever seen the Milky Way, that faintly glowing cloud 

 of matter that stretches across the heavens? To the 

 naked eye it has a milky appearance because of the ap- 

 parent nearness of the stars to each other, but when 

 looked at through a telescope it becomes a huge group 

 of millions of stars (see Fig. 286). While the stars 

 that make up the Milky Way seem very close to each 

 other, they are in reality separated from each other by 

 millions of miles. 



